Benefits of Molasses for the Garden? Soil Priming! [Commentary]

Not scientific? Master Gardener? *Alarm bells*

Glucose, a six-carbon sugar is one of the sugars in molasses. It can prime carbon in the soil, increasing soil organic matter decomposition by bacteria that make it more plant available as dissolved organic matter.

Priming or a “Priming Effect” is said to occur when something that is added to soil or compost affects the rate of decomposition occurring on the soil organic matter (SOM), either positively or negatively. Organic matter is made up mostly of carbon and nitrogen, so adding a substrate containing certain ratios of these nutrients to soil may affect the microbes that are mineralizing SOM. Fertilizers, plant litter, detritus, and carbohydrate exhudates from living roots, can potentially positively or negatively prime SOM decomposition

Look at fig (3c) for organic soil here:

The red indicates positive carbon priming when glucose is added.
Notice how different the glucose priming effects are for wood (a), leaf (b), organic soil (c) and mineral soils (d) in each of the squares. Think about why that might be, and where the carbon-based lifeforms are most abundant. The organic soil!

Priming is how organic soils are formed when plants exude carbon compounds from their roots to feed soil microbes.

Unfortunately not all soils provide enough nutrients to the plants to create enough exudates to prime soil carbon and maintain nutrient cycling, and soil carbon is mostly determined by rainfall (78%) and how land is managed, which is why we tend to augment soils with fertilizers that perform this priming.

As shown in fig (3c), priming soils and composts with nitrogen can also aid this process, an example of priming compost are these “dreadlock” roots formed when using “Next Gen” compost that adds nitrogen and other minerals.

Priming, however it’s done, generally results in higher dissolved organic matter and microbial abundance, diversity and nutrient cycling resulting in more plant available nutrients.

Soil carbon to nitrogen ratios can determine whether carbon or nitrogen is the best choice to prime organic matter and will depend on the soil and the optimum range that soil microbes like fungi that knit the soil food web together like to feed on. Typically that’s the C:N of 30:1 that microbes are made of. Probably why there’s a tipping point at 3% soil carbon. Many bare and underperforming soils are well below this and crave carbon. Priming soil organic carbon is how biochar works so long as you add enough

The type of carbon or nitrogen source when priming is important too, as it may alter microbial communities. The more complex the carbon source the more potential there may be for enzymatic pathways that the microbes can express to create compounds that change their environment.

Different species of plants change their own environments by exuding different exudates that host different microbes that build the environment for them.

Doing the work of the plant by amending soils ourselves may benefit or hinder these microbes.

It’s important to note that many types of molasses are heavily processed and end up with sugars but very few minerals in them, and this may change the microbial community detrimentally.

What gets added to white sugar to make it brown? Molasses.

You can see it does contain and add some minerals.


In general, the less processed something is, the more minerals it contains, and the more diversity it will support, thereby allowing the plant to feed and select for the microbes it wants through its exudates rather than what will eat what we amend the soils with.

Cultivate those soil microbes with carbon where appropriate.

Residue Amendment and Soil Carbon Priming for Richer or Poorer

Feed the microbes carbon in C-poor soil and they’ll have a party.
Feed the microbes carbon in C-rich soil and they’ll put it in the C-bank.
This quote is of particular interest, emphasis mine:

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

Predator-prey carbon sequestration? Sounds similar to the Arthropod predator results. May the shift be with you.

The study:

A 150-day incubation experiment was conducted with 13C-labelled soybean residue (4%) amended into two Mollisols differing in SOC (SOC-poor and SOC-rich soils). …

The amounts of residue-C incorporated into the coarse particulate organic C (POC), fine POC and mineral-associated C (MOC) fractions were 4.5-, 4.3– and 2.4-fold higher in the SOC-rich soil than in the SOC-poor soil, respectively.

Residue amendment led to negative SOC priming before Day 50 but positive priming thereafter.

The primed CO2 per unit of native SOC was greater in the SOC-poor soil than in the SOC-rich soil. This indicates that the contributions of residue-C to the POC and MOC fractions were greater in the SOC-rich soil while residue amendment had stronger priming effect in the SOC-poor soil, stimulating the C exchange rate between fresh and native SOC.

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

The fate of soybean residue-carbon links to changes of bacterial community composition in Mollisols differing in soil organic carbon